Flyback Power Supply Powers Subscriber Line Interface Circuit

Cable and xDSL modems are experiencing increasing popularity. For this reason, many designs are now required to interface with existing telephones at the subscriber`s location. The subscriber line interface circuit (SLIC) within the modem has the additional burden of ringing the phone as well as providing loop current while a conversation is takin
Flyback Power Supply Powers Subscriber Line Interface Circuit - schematic

g place. While the phone is ringing and "on-hook, " it appears as an 8k resistance in series with another 1k of capacitive reactance. Typically, the SLIC must be capable of driving this impedance with a 45-VRMS, 20-Hz sinewave with a negative dc-offset in order to ring the phone. Therefore, a high voltage of between ’50 and ’105 V is usually mandated by the SLIC. Once the handset is lifted, the phone places a much lower impedance across the phone terminals, and the SLIC goes into a 20-mA constant-current mode. As a result, the SLIC only needs a power supply of ’24 V. Figure 1 shows the schematic of the power supply for the SLIC portion of the modem. This circuit provides multiple outputs from a single power switch and control IC. It also uses an efficient n-channel MOSFET with low voltage stress. A low input voltage powers a flyback topology. The input source could be either a widely differentiating (generally 2. 5 to 1) output from an ac adapter, or a regulated supply used by some other portion of the system. U2 is the brain of the power supply, as it modulates Q2`s duty factor to control the output voltage. Also, it produces a reference voltage inverting amplifier U1 uses to generate an error signal. The control IC can perform either in current-mode or voltage-mode control. A key advantage of the multi-winding approach is that a single control circuit and single MOSFET can supply two telephony voltages. Good...

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